1. Field of the Invention
This invention relates to an inspection device for inspecting projections on the surface of parts. More particularly, the present invention relates to a lead inspection device suitable for inspecting the lead of surface mount type electronic parts.
2. Description of the Prior Art
A typical example of surface mount type electronic parts to be mounted to a printed circuit board is a so-called PLCC (Plastic Leaded Chip Carrier).
In this PLCC, leads extending from the side surfaces of a package are bent in J shape towards the bottom view side of the package. This PLCC is mounted to the printed circuit board with the bottom view side of the leads facing downward and the leads are soldered to pads of the printed circuit board.
The surface mount type electronic parts of the kind described above can increase more highly the mount density in comparison with so-called axial lead type electronic parts wherein the leads are inserted into through-holes of the printed circuit board. However, since the solder portion of each lead is hidden below the parts after soldering, it is difficult to look in parts failure.
Solder failure results mostly from the abnormal outward form of the lead caused by pushing up the lead or pushing down the lead, such as curve and float of the lead.
It is therefore important to inspect the normality of the lead before mounting the surface mount type parts such as a PLCC to the printed circuit board and to reject in advance the parts having abnormal leads in order to reduce the failure of soldering and to improve reliability.
FIG. 5 of the accompanying drawings is a schematic view of a conventional lead inspection device for inspecting the normality of the lead of surface mount type electronic parts such as a PLCC in which the leads are bent in the J shape.
In the conventional device, the part 1 to be inspected is fixed on a flat table 2 with the bottom view side of the lead facing upwards. The flat table 2 is then raised to a predetermined height by a cylinder 3 to locate the part 1 to a predetermined position of a light projector/receptor and to make inspection of the lead.
In this light projector/receptor system, the output light of the projector 28 is guided to the center portion of the bottom view side of the part 1 through a glass fiber 35, then emitted outside, received by glass fibers 36 that are disposed outside in such a manner as to corresponds to the leads of the part 1, respectively, and guided to the receptor 27 corresponding to the lead. Therefore, the quantity of incident light to the receptor 27 changes in accordance with the abnormal outward form of the leads such as float and/or curve.
The output signal of the receptor 27 is converted to a digital signal by an analog-to-digital convertor 29 and inputted to a decision unit 30. This decision unit 30 compares the value of the digital signal corresponding to each lead with the value of a digital signal corresponding to the same lead obtained from a good sample under the same condition in order to make decision of the lead of the part 1.
As an example of the prior art references relating to the inspection device of this kind, mention can be made of "MI-8500A PLCC Inspection System Catalog" of Machine Intelligence Company.
In accordance with the conventional device, however, the projector/receptor and good part samples must be changed whenever the surface shape of the parts such as the size and shape and the number of leads change, and cannot easily cope with many kinds of parts.
Moreover, the detection error is likely to occur because the conventional inspection system is based on the principle that the abnormal outward form of the lead such as the curve and/or float is judged by comparing the quantities of incident light into the projector corresponding to the individual lead.